446 BROOKLYN BOTANIC GARDEN MEMOIRS 



that natural selection has presided over the appearance of these new 

 forms, for groups which have developed few or no endemic species are 

 apparently as successful elements of the vegetation as are those in 

 which such species have been abundantly produced. In fact, Willis^ 

 has gathered evidence from the flora of Ceylon which seems to show 

 that the non-endemic species are more successful, as a whole, than the 

 endemic ones, a fact which militates strongly against the theory of 

 selection. Of course we are confronted here, also, with one of the 

 major difficulties urged against natural selection, namely that it can 

 never create but can only eliminate. 



Nor do our figures support the theory that local forms owe their 

 origin to the direct action of the environment, for such a theory can- 

 not well explain the abundance of endemic species in some groups 

 and their rarity in others. It may be argued that the vascular crypto- 

 gams and glumaceous monocotyledons are more primitive and slow- 

 changing types than the petaliferous groups, and are thus able longer 

 to resist the pressure of the environment and to maintain their original 

 characters. We have little evidence, however, that this is actually the 

 case. Ferns under cultivation seem to be very plastic, and our knowl- 

 edge of the genetics of the Gramineae, at least, does not indicate that 

 they are a particularly rigid group. 



Both of these views look to the environment as the factor, either 

 direct or indirect, which is chiefly responsible for the origin of new 

 forms, and both are open to the objection (among others) that although 

 the whole flora is subject to the same environment, these new forms 

 develop only in certain groups. Our third alternative largely dis- 

 regards the environment. It looks upon the actual production of 

 new types as due to factors within the organism rather than in its 

 surroundings, and considers that the locally developed species and 

 genera in the floras under discussion would have appeared in these 

 regions whether isolation existed or not. Isolation is thus regarded 

 merely as the agency which keeps these new forms local and endemic 

 by preventing their dispersal beyond the place of their birth. Of 

 course such a theory allows for the play of selection in weeding out all 

 new forms which were distinctly unsuited to the environment under 

 which they appeared. 



But is not this view also open to the objection which we have 

 offered to the others, that it cannot account for the rarity of endemism 

 in certain groups and its extreme commonness in others? A study of 

 the methods of reproduction in plants belonging to these two cate- 

 gories suggests an answer to this question. Vascular cryptogams in 

 the great majority of cases have bisexual gametophytes and are 



2 Willis, J. C, The evolution of species in Ceylon, with reference to the dying 

 out of species. Annals of Botany 30: 1-23. 1916. 



